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Atomically Thin CrCl3: An in-Plane Layered Antiferromagnetic Xinghan Cai,1,2 Tiancheng Song,1 Nathan P. Wilson, 1 Genevieve Clark,3 Minhao He,1 Xiaoou Zhang,4 Takashi Taniguchi,5 Kenji Watanabe,5 Wang Yao,6 Di Xiao,4 Michael A. McGuire,7 David H. Cobden,1 Xiaodong Xu1,3*

1Department of , University of Washington, Seattle, Washington 98195, USA. 2National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Department of Micro/Nano Electronics, Shanghai Jiao Tong University, Shanghai 200240, China. 3Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, USA. 4Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA. 5National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan. 6Department of Physics and Center of Theoretical and Computational Physics, University of Hong Kong, Hong Kong, China. 7Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.

*Correspondence to: [email protected]

Abstract: The recent discovery of in atomically thin layers of van der Waals (vdW) crystals has created new opportunities for exploring magnetic phenomena in the two- dimensional (2D) limit. In most 2D studied to date the c-axis is an easy axis, so that at zero applied field the polarization of each layer is perpendicular to the plane. Here, we demonstrate that atomically thin CrCl3 is a layered antiferromagnetic insulator with an easy- plane normal to the c-axis, that is the polarization is in the plane of each layer and has no preferred direction within it. Ligand field photoluminescence at 870 nm is observed down to the monolayer limit, demonstrating its insulating properties. We investigate the in-plane magnetic order using tunneling magnetoresistance in graphene/CrCl3/graphene tunnel junctions, establishing that the interlayer coupling is antiferromagnetic down to the bilayer. From the temperature dependence of the magnetoresistance we obtain an effective magnetic diagram for the bilayer. Our result shows that CrCl3 should be useful for studying the physics of 2D phase transitions and for making new kinds of vdW spintronic devices.

Keywords: 2D magnetic insulator, in-plane layered antiferromagnetism, magnetic tunnel junction, weak , magnetic

The experimental observation of magnetism in atomically thin films1-8 has opened a new avenue to study novel magnetic multilayered devices using 2D materials9-21. To date, all magnetic compounds that have been successfully synthesized down to the monolayer limit exhibit strong Ising anisotropy, which supports an out-of-plane magnetization within each layer. In this regard, CrCl3 represents a curious case in that bulk CrCl3 is known to be a weakly coupled, layered 22-25 antiferromagnet with small XY anisotropy (Figure 1a) . Therefore, exfoliating CrCl3 offers the intriguing possibility to realize a strictly 2D magnetic insulator with in-plane magnetization. This could be useful for investigating how a with weak anisotropy evolves on approaching the 2D limit. The monolayer may behave as a good approximation to the 2D XY-model, analogous to thin film superfluids, superconductors and crystals26,27, allowing the study of the Berezinskii–Kosterlitz–Thouless (BKT) transition28 and related phenomena. Moreover, the in- plane magnetization is also complementary to the out-of-plane magnetization in terms of the 29-33 proximity effect when CrCl3 is layered with other 2D materials.

CrCl3 flakes were exfoliated onto SiO2/Si substrates. In atomic force microscope images (Figure 1b), bilayers and trilayer regions appear 1.2 nm and 1.8 nm thick, respectively, consistent with a previous report25. Figure 1c shows both the circular polarization components of the photoluminescence (PL) from a ~ 5 nm thick sample at a temperature of 2 K, well below the Neel temperature of ~ 14 K. The excitation laser is 20 µW, linearly polarized, and at 532 nm. A single broad peak centered at ~ 870 nm is seen, analogous to the parity-forbidden d-d ligand-field 34 transition in CrI3 . Unlike in CrI3, where the out-of-plane magnetization component results in spontaneously circularly-polarized PL, the two polarization components in thin bulk CrCl3 are indistinguishable, consistent with the in-plane magnetization determined in its bulk crystal. This PL emission feature persists down to the monolayer CrCl3 (Figure 1d), implying that the insulating nature retains in CrCl3 to its 2D limit. To investigate the in-plane magnetization we employ vertical tunneling measurements, which are sensitive to the relative alignment of spins in different layers. As sketched in Figure 2a, a few layer CrCl3 tunnel barrier is sandwiched between top and bottom thin graphite flakes to form magnetic tunnel junction (MTJ), this being further encapsulated in hexagonal boron nitride (h-BN) to avoid degradation (see Methods). The tunneling junction area is typically less than ~1 μm2 to avoid 1,13 effects caused by lateral structures . Figure 2b shows the tunneling current, It, as a function of DC bias voltage, V, measured at 2 K for a bilayer CrCl3 device whose optical image is inset. With an applied in-plane μ0Hin=2 T (red trace) the current is enhanced relative to its value at zero field (blue trace), implying a change in the configuration 35 of the CrCl3 . The bottom inset shows the tunneling magnetoresistance (TMR), defined as 13 100% × [퐼푡(2 T) − 퐼푡(0)]/퐼푡(0) . The TMR is largest at low bias, where the response is linear. Figure 2c shows the tunneling current, It, here measured using a small AC bias of Vac = 1 mV, as a function of magnetic field. For either in-plane field (퐻푖푛, orange) or out-of-plane field (퐻표푢푡, c green), It increases smoothly before plateauing out when the field exceeds a critical value (μ0Hin c = 1.2 T and μ0Hout = 1.5 T). This behavior is similar to that seen for an in-plane field in CrI3 MTJs. It implies that the spin polarizations of the two layers are opposite at zero field, suppressing the tunneling current by a spin-filtering effect, but become increasingly aligned due to the Zeeman term in an external field. This in turn suggests that the layered antiferromagnetic order of the bulk crystal, with in-plane ferromagnetic polarization that alternates in direction between adjacent layers, persists in the bilayer limit. c c The fact that Hout is only slightly larger than Hin , and the smooth rise of 퐼푡 seen for both field orientations, are consistent with relatively weak anisotropy in the material. Indeed, the field dependence of the tunneling current can be well fit by a simple model of two antiferromagnetically coupled macrospins with easy-plane anisotropy, 퐾 퐻 = 퐽 풔 ⋅ 풔 + (푠2 + 푠2 ) − 푔휇 푩 ⋅ (풔 + 풔 ), 퐴퐹 1 2 2 1,푧 2,푧 퐵 1 2 where 퐽퐴퐹 > 0 is the interlayer antiferromagnetic exchange, 퐾 > 0 is the easy-plane anisotropy energy, and 푩 is the magnetic field. According to our fitting, the anisotropy is 퐾푆/푔휇퐵 = 0.27 T and interlayer coupling is 퐽퐴퐹푆/푔휇퐵 = 0.6 T (See Supplementary material). We note that there is some asymmetry of It as a function of 퐻푖푛, associated with hysteresis as shown in Figure 2d. This might be due to a spin-flop transition associated with a previously reported, very small easy-axis 24,25 anisotropy (10 Oe) within the easy-plane . No hysteresis is seen as a function of Hout (Figure 2d, inset), as expected for spin canting in a magnetic field perpendicular to the zero-field polarization. This is in sharp contrast to the situation in bilayer CrI3, where the transition from antiferromagnetic to a fully aligned ferromagnetic state in a perpendicular field is a sudden spin- flip transition. The picture of layered antiferromagnetism is further supported by measurements on a trilayer CrCl3 MTJ device (Figure 3), an optical micrograph of which is inset to Figure 3b. The thicker barrier presented by a trilayer leads to much weaker tunneling which necessitates a much larger bias voltage to achieve a suitable signal to noise ratio. Without loss of generality, a DC bias of 600 mV is applied (See supplementary material for the bias voltage dependent tunneling measurements). As seen in Figure 3a, It increases smoothly with Hout, again consistent with gradual 13 spin canting out of the layers as in the bilayer case . However, as a function of 퐻푖푛 the current exhibits a plateau near zero field below a critical value, above which it smoothly rises and reaches a plateau when the magnetic field is large enough to align all three layers to the fully spin polarized state. To test for magnetic anisotropy within the plane of the layers, we measured the tunneling current while varying the orientation, θ, of Hin within the plane. Figure 3b shows It as a function of Hin at selected angles, while Figure 3c is a polar plot of It as a function of angle at selected magnetic fields. No significant dependence on 휃 is observed, implying that the overall magnetic order can rotate freely about the c-axis, i.e., the plane of the layers is an easy plane. The observed plateau of tunneling current as a function of in-plane magnetic field is due to the uncompensated total magnetization in a trilayer, consistent with the picture of layered antiferromagnetism. We conclude that atomically thin CrCl3 can be well described as a layered antiferromagnetic insulator with easy- plane magnetic anisotropy.

Figure 4a shows the tunneling current It measured AC with Vac = 5 mV as a function of T at selected in-plane magnetic fields. At Hin = 0, on increasing 푇 from base, at first It increases. This can be explained by suppression of the spin filtering effect due to thermal fluctuations which reduce the degree of antiferromagnetic alignment between the layers. Then, above a peak at the intralayer ferromagnetic critical temperature 푇~17 K, 퐼푡 starts to decrease. This suggests that ferromagnetic order within each layer assists coherent tunneling, and rising temperature suppresses this order. At still higher temperatures 퐼푡 increases again, as expected since once no magnetic order remains the tunneling rate should simply be activated. When an in-plane field is applied it competes with the antiferromagnetic alignment and reduces the temperature at which the AFM order appears, so the peak moves to lower 푇. It also increases net spin alignment above the critical temperature and so increases the current. For μ0Hin > 1.2 T, the antiferromagnetism is completely suppressed and the spins are fully polarized at base temperature.

The results in Figure 4a can be presented in a more visualized way: dIt/dT has been plotted as a function of the temperature at selected μ0Hin (Figure 4b). In the top panel, dIt/dT clearly exhibits two times of the sign flip as T increases. This is similar to what has been reported in CrCl3 3D bulk crystal25, which is interpreted as a two-step phase transition, i.e. from the layered antiferromagnetic phase to the ferromagnetic phase (first transition) and eventually to the paramagnetic phase (second transition) as T increases. Here we assign the peak (directed by the green arrow) in each curve to the critical temperature of the first transition and the dip (directed by the blue arrow) to the second one. For magnetic fields higher than 1.2 T, as shown in the bottom panel, no local maximum shows up, thus only Tc for the second transition is determined. In Figure 4c, we present the 2D semi-log plot of the normalized dIt/dT as a function of both μ0Hin and T (see supplementary material for the plot without the normalization of dIt/dT). This plot, though not a legitimate phase diagram, facilities comparison with the phase diagram of the 3D bulk crystal in which distinct transitions are identified between the layered AFM phase, a fully spin-polarized phase, and paramagnetic phase25.

To recap, we find that the van der Waals magnetic insulator CrCl3 shows layered antiferromagnetism down to the bilayer limit with the magnetic moments in the plane of the layers and little or no anisotropy within the plane. CrCl3 thus provides a new model system for studying 2D in-plane magnetism and its proximate coupling to other 2D materials, such as 2D superconductors, topological insulators, and stacked heterostructures30,36 for potential use in novel magneto-electronic devices13-18. During the final preparation of the manuscript, we became aware of the two relevant works posted on ArXiv37,38. METHODS

Device fabrication: V/Au (4 nm/40 nm) metal electrodes were deposited on a 90 nm SiO2/Si substrate in the vicinity of the exfoliated h-BN thin flake (bottom h-BN), following the standard e-beam lithography procedures. Bulk CrCl3 crystals were grown by chemical vapour transport, as 25 described in detail in ref . Few layer CrCl3, cleaved from these crystals in an inert glovebox with oxygen and water levels below 1 ppm, was stacked between the pre-exfoliated top h- BN/graphite and bottom graphite and transferred onto the bottom h-BN using a -based dry transfer technique39. Top/bottom graphite flakes were contacted by the pre-patterned metal electrodes and the tunnel junction was then fully encapsulated before finally dissolving the polymer in chloroform outside the glovebox. The thickness of the h-BN (5-35 nm) and graphite (2-8 nm) were determined by atomic force microscopy, while the number of layers of CrCl3 was identified from the optical contrast. Optical measurements: Low-temperature magneto-PL measurements were performed in a closed-cycle cryostat with a superconducting magnet with field normal to the sample. The sample was excited by a continuous-wave laser (633 nm or 532 nm) with the power below 50 μW to avoid sample heating and degradation. The PL signal was detected by a silicon CCD array and the excitation and detection polarization were controlled using linear polarizers and half- and quarter- wave plates. Electrical measurements: All magneto-transport measurements were carried out in a PPMS DynaCool cryostat (Quantum Design, Inc) with a base temperature of 1.8 K and magnetic field up to 9 T. The CrCl3 MTJ devices were mounted in a Horizontal Rotator probe using either a Rotator Universal Sample Board (P103C), which allows rotations about an axis in the sample plane perpendicular to the magnetic field, or a Rotator Field Parallel Sample Board (P103D) for rotation about the magnetic field axis. The bias voltage is applied to the top graphite contact while the current from the bottom graphite contact is measured using a virtual-earth current preamplifier (DL Instruments; Model 1211). AC measurements were made using a lock-in amplifier (Stanford Research 830) at 13.7 Hz.

Figure Captions:

Figure 1. CrCl3 crystal structure and the optical characterization. (a) Side view of the structure of CrCl3. The colored balls represent Cr (blue) and Cl (grey), respectively. Red arrows indicate the magnetization. (b) Atomic force micrograph of few layer CrCl3 flakes. The inset is an optical micrograph of the same area. (c) Left and right circular polarization components of photoluminescence from a 5 nm thick flake under a linearly polarized excitation at 532 nm. (d) Photoluminescence spectrum of a monolayer CrCl3.

Figure 2. Magnetic field dependent tunneling in a bilayer CrCl3 spin-filter magnetic tunnel junction (sf-MTJ). (a) Schematic of device with graphite contacts encapsulated by h-BN. (b) Tunneling current as a function of the DC bias voltage with (red) and without (blue) an in-plane magnetic field μ0Hin=2 T. Top inset: optical microscope image of the device with false coloring for clarity. Bottom inset: TMR ratio. (c) Tunneling current It as a function of in-plane (orange) and out-of-plane (green) magnetic field for bias Vac = 1 mV. The corresponding magnetic states are indicated. (d) Tunneling current vs in-plane magnetic field sweep in both directions. The magnetic configurations are indicated. Top inset: Tunneling current vs out-of-plane field swept in both directions, showing no hysteresis.

Figure 3. Magnetic field dependent tunneling in a tri-layer CrCl3 sf-MTJ device. (a) Tunneling current It as a function of the in-plane (orange) and out-of-plane (green) magnetic field at Vdc = 600 mV. The corresponding magnetic states are indicated. (b) Tunneling current as a function of in-plane magnetic field at selected orientations θ (here Vdc = 750 mV). The inset is a false-color optical micrograph of the device with θ defined. (c) Polar plot of the tunneling current as a function of θ at selected magnetic fields.

Figure 4. Temperature dependent tunneling and the magnetic phase diagram of bilayer CrCl3. (a) Tunneling current It as a function of the temperature at selected in-plane magnetic fields with the AC bias voltage Vac = 5 mV. (b) The derivative of the tunneling current to the temperature at low (top panel) and high (bottom panel) in-plane magnetic fields. The green/blue arrows indicate the critical temperature of the transition from layered antiferromagnetic/spin polarized to spin polarized/paramagnetic states, respectively. (c) Color plot of normalized dIt/dT as a function of in-plane magnetic field Hin and temperature T. AFM: Layered antiferromagnetic. PM: Paramagnetic. SP: Fully spin polarized.

Acknowledgments: Funding: The work at UW is mainly supported by NSF MRSEC 1719797. Device fabrication and transport measurements are partially supported by NSF-DMR-1708419. Photoluminescence measurement is supported by DOE BES DE-SC0012509. Work at HKU is supported by the Croucher Foundation (Croucher Innovation Award), RGC of HKSAR (17303518P). Work at ORNL (M.A.M.) was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, A3 Foresight by JSPS and the CREST(JPMJCR15F3), JST. D.X. acknowledges the support of a Cottrell Scholar Award. X.X. acknowledges the support from the State of Washington funded Clean Energy Institute and from the Boeing Distinguished Professorship in Physics. Author Contributions: X.X., X.C., M.A.M. conceived the project. X.C. fabricated the devices, performed the experiments, and analyzed the data, assisted by T.S. and M.H. N.P.W. and G.C. performed photoluminescence measurements. X.Z. analyzed the macrospin model and the tunneling current dependence on the . X.X., D.X., W.Y., and D.H.C. supervised the project. M.A.M. provided and characterized bulk CrCl3 crystals. T.T. and K.W. provided and characterized bulk hBN crystals. X.C., X.X., D.H.C. and D.X. wrote the manuscript with input from all authors. Competing Financial Interests: The authors declare no competing financial interests. Data Availability: The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supporting Information: Additional measurements on the bilayer CrCl3 MTJ and modelling of the tunneling current; Characterization and discussion of the magnetic properties of multilayer and monolayer CrCl3; PL measurement on monolayer CrCl3.

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